1. Field of the Invention
The present invention relates to a method of transmitting data in an ad hoc network or a sensor network with the network including a multitude of sensor nodes to sensor data, at least one aggregator node to aggregate the sensored data of at least a subset of the sensor nodes, and at least one sink node to collect the data from the aggregator nodes.
2. Description of the Related Art
Regarding the development and the ever growing increase of ad hoc networks and sensor networks respectively, the present method of transmitting data within these networks is of an ever increasing importance. One of the major requirements within wireless ad hoc networks and sensor networks is the aggregation of sensored data and its transport to a specific device.
All sensors of a sensor network are sensor nodes which communicate with each other in a wireless way and which consist in general of a probe, a processing unit, a communication device and a battery. The sensor nodes comprise the functionality of data acquisition, communication and computation on a minimum of space. To provide examples where sensor networks are used, monitoring and controlling machines, controlling (intra- and extra-corporal) health parameters or environment monitoring should be mentioned here. The range of application possibilities for sensor networks is almost infinite, though. In specific fields, such as examining the contamination of water or weather forecasting, for example, it is extremely advantageous that sensor nodes can be realized in miniature size and that they can easily be fixed and used in regions hard to access.
Critical parameters restricting under certain circumstances the application possibilities of sensor networks are in particular physically defined factors of the single sensor nodes, such as, for instance, the reach of their sender, processor power, battery capacity, existing storage space and the like. Since the single sensor nodes—in contrast to the sink node where the sensored data comes together—are physically restricted in many ways, the energy-efficient organization of the sensor network is of outstanding importance. In this context it first has to be stated that the transmission of all the sensored data to the sink node would cause by far too much data traffic, so the data is usually accumulated within the network at special nodes—the aggregator nodes—first. Sending all the sensored data to its final destination would result in a lifetime which would be unacceptably short since the energy consumption of the devices, i.e. the sensor node, during sending increases in a linear way with the amount of data to send.
Another important aspect which has to be taken into consideration when establishing a sensor network is a secure transmission of data. The platforms which form the base of the sensor nodes usually have a specific miniature design and it can not be assumed that they dispose of a tamper-resistant unit. Thus, a subset of sensor nodes of the network may be corrupted. The aggregator nodes, in particular, being intended for collecting data from sensor nodes of their neighborhood, are an attractive aim for attacks because there the sensored information of a whole region is available and consolidated. After an aggregation function has been executed on the received sensored data, the aggregator nodes transmit their results in a compressed form to the sink node.
FIG. 1 shows—as a scheme—a sensor network with a multitude of sensor nodes 1 being labeled according to their number by S1 to Sn. After a request from the sink node 2—which is a specific device with sufficiently big physical resources—, the n sensor nodes, S1 to Sn, send their sensored data, s1 to sn, to an aggregator node 3 in their neighbourhood.
For reasons of clarity, in FIG. 1—only one single aggregator node 3 is shown. The aggregator node 3 executes the aggregation function y=f (s1, s2 . . . , sn) on the sensored data S1 to Sn, and then sends the result y of the aggregation function f to the sink node 2.
In the scenario shown no encryptions take place at all, so a potential attacker can easily tap the single values s1 to sn as well as the sensored information of a whole region, i.e. the result y of the aggregation function f.
For some applications security concepts of some kind are a mandatory pre-requisite. Already existing approaches to enhance security of data transmission frequently bring about additional and too heavy data traffic on the wireless medium which renders an economic use in practice often impossible. Using simple hash functions, for example, creates eight bytes as additional data per packet. Considering the fact that in sensor networks usually current radio technology is utilized with a maximum packet size of 36 bytes and a maximum payload of 27 bytes, it becomes evident that even the usage of hash functions should be evaluated carefully. This example explains once more the sensitivity of utilization of current methods, regarding security aspects as well as maximum energy saving. All of the methods known have in common that their realization—regarding aspects of security related to data transmission—consumes too much energy which makes the utilization of secured sensor networks in practice uneconomical.